Where is the missing carbon?

A giant hole in the global carbon budget may be plugged by an unlikely source: fish guts.

A large proportion of manmade CO2 emissions drain back out of the atmosphere into various carbon sinks. Scientists have long known that approximately half the CO2 flux from the atmosphere goes to land-based sinks and half to the ocean. The problem is that the math hasn’t quite added up.

Terrestrial sinks like forests and savannas, in addition to the long-term storage in soils, are relatively well quantified. Primary absorption in the oceans, too, has been fairly well described: satellite imagery of the open ocean has been used to map and calculate the amount of primary absorption across 70% of the earth’s surface.

Still, the numbers contain a substantial gap. Since the terrestrial sinks are comparatively better understood, and the CO2 in the atmosphere has to go somewhere, most scientists assumed that it was somehow ending up in the ocean. But where?

Almost twenty years ago, researchers at the University of Miami discovered that a species of toadfish carries tiny balls of calcite (CaCO3) in its gut. The authors suggested that this was likely a result of a filtration system in the fish’s stomach: water breathed in and out by the fish would need to be cleaned of various salts, including calcium and magnesium, to maintain proper salinity. These salts combine with carbon in seawater to form carbonates, which precipitate and collect in the fish’s gut.

It turns out that toadfish aren’t unique. All bony fishes have this feature. A new study calculates that these tiny calcite stones could be a missing sink that accounts for 3-15% of the oceanic carbon absorption. That’s a big hole to plug, and the study’s figures are conservative. The actual number could be significantly higher.

This provides another reason to be concerned with declining fish stocks worldwide. In addition to missing out on your favorite tuna sandwich, the global fisheries collapse could end a vital sink for atmospheric carbon.